The present invention relates to a product based on calcium oxide in the form of coarse grains and a method of manufacturing such a product.
In the present invention, product based on calcium oxide means not only quick lime CaO, but also unhydrated dolomite CaO.MgO, and all intermediaries xCaO. (1xe2x88x92x)MgO, where x is equal to or less than 1 but greater than 0.
These products based on calcium oxide in the form of coarse grains are in the form of grains whose size is greater than 10 mm, and is preferably between 10 and 70 mm.
The present invention therefore does not relate to the processing of lime in powder form or with fine grains, as provided for for example in JP-58180225, JP-56050115 and JP61261242.
It is possible to obtain products based on such coarse grains by baking a raw material, such as for example limestone, and then screening the product obtained, known as rock lime. Rock lime according to the invention means not only this, but also rock dolomite, and intermediates such as dolomitic rock lime. It is also possible to obtain products based on calcium oxide in the form of coarse grains within the meaning of the invention by compacting fine particles of quick lime, unhydrated dolomite or the aforementioned intermediates, and forming from this material small briquettes or tablets of the size indicated above.
Rock lime generates fines at the various steps of its handling, from its removal from the furnace at the end of the baking of the raw material until it is used by the user, for example as an additive in steel making converters. The fines are produced by fragmentation of the lime grains, when they fall during the loading or unloading of a lorry or silo, or by attrition during friction between the lime grains during transportation by lorry or on conveyer belts.
Likewise the aforementioned briquettes based on calcium oxide refragment during their handling and more or less rapidly reform a new fraction of fines.
Fines, within the meaning of the invention, means any particle whose size is less than the nominal size of the product used. For example, if the user requires a product with a granulometry from 10 to 70 mm, the fines will be formed from particles with a size of less than 10 mm. In a case of this type, the proportion of fines (0 to 10 mm) represents xc2x13% of the mass of rock lime, at the discharge from the furnace. It may attain 8 to 10% after unloading at the user. If for example a steelmaker is involved, the proportion of fines may reach 15 to 25% at the time of feeding into the converter. When these fines fly off during the transportation and handling of the lime, this represents not only a considerable economic loss, but also constitutes a not insignificant source of pollution, whose treatment is becoming more and more expensive.
It is obviously possible to reduce the emission of fines by screening operations and an improvement to infrastructures, in order to reduce the height of the falls.
The aim of the present invention is to develop a product based on calcium oxide in the form of coarse grains which offers improved mechanical strength and consequently a reduction in the attrition of material and its fragmentation by impacts and therefore a reduction in the emission of dust during the handling thereof. Another aim of it is to produce a method of manufacturing a product based on calcium oxide in the form of coarse grains which improves the said mechanical strength of the product. Advantageously, the product according to the invention must be able to be in a form impermeable to moisture in order to improve its storage properties.
These problems have been resolved, according to the invention, by a product based on calcium oxide in the form of coarse grains which has a protective film coating the grains, based on at least one film-forming compound, which is solid at room temperature and which, during application to the grains, is in solution or emulsion in a vaporisable aqueous phase.
The present invention therefore consists of acting on the material by coating grains with a protective film which will reduce the phenomena of attrition and fragmentation of the grains, this film solidifying when the aqueous phase evaporates.
According to an advantageous embodiment of the invention, the said aqueous phase is water and the product also comprises, on the surface of the grains, a thin layer of hydrated lime which is coated with the said hardened protective film.
A reaction of hydration of CaO into Ca(OH)2 occurs at the surface of the grains based on calcium oxide during the application of the aqueous solution or emulsion of the film-forming compound. This reaction is sufficiently exothermic to raise the temperature locally, up to about one hundred degrees Celsius, and to almost instantaneously evaporate the water which has not yet reacted, which affords a very rapid hardening of the surface protective film. The speed of the coating is favourable in an industrial process since it reduces the time taken for obtaining an easily handleable product. In addition, the water, in evaporating, does not emit any harmful, flammable or explosible vapours and therefore represents an entirely preferential liquid phase. It should also be remarked that thin layer of hydrated lime means according to the invention not only a layer of pure hydrated lime but also hydrated lime partially containing hydrated magnesia.
The film-forming compounds according to the invention must be soluble or capable of forming an emulsion in an aqueous phase and be capable of hardening after the evaporation thereof, forming a solid phase which, at room temperature, coats the grains. As film-forming compounds of this type it is in particular possible to cite, by way of non-limiting examples, vinyl homopolymers or copolymers, such as polyvinyl alcohol, polyvinyl acetate, a vinyl ethylene-acetate copolymer, acrylic homopolymers or copolymers, such as C1-C18 alkyl esters of acrylic acid or methacrylic acid, paraffins, notably short-chain paraffins, homopolymers or copolymers of butadiene, isoprene and styrene, silicones or mixtures of these compounds. A vinyl ethylene-acetate copolymer has proved to be particularly favourable as a film-forming compound according to the invention. In particular silicones or polyisoprene are envisaged for producing grains which are not only mechanically strong but also relatively impermeable to water.
Advantageously, in the hardened state, this film-forming compound forms around the grains a film of protective coating which has a thickness of less than 100 xcexcm.
Details and particularities of the product according to the invention are also indicated in the claims which follow.
The invention also concerns a method of treating a product based on calcium oxide in the form of coarse grains.
According to the invention, this method comprises
an application, on the surface of the said grains obtained, of a solution or emulsion in an aqueous phase of at least one film-forming compound, solid at room temperature,
evaporation of the aqueous phase, and
hardening of a protective film of the said at least one film-forming compound, coating the grains.
This method is particularly simple, inexpensive to implement, and has above all the advantage of low consumption in a coating environment. Advantageously, after evaporation of the aqueous phase, a protective solid film is obtained with a thickness of only a few tens of micrometres.
According to a preferred embodiment of the invention, the method comprises, during the above mentioned application, an exothermic reaction between part of the aqueous phase and the calcium oxide contained in the grains, formation of a thin layer of hydrated lime on the surface of the grains, and instantaneous evaporation of the remaining aqueous phase caused by an increase in temperature resulting from the said exothermic reaction, which gives rise to a rapid hardening of the said protective film.
According to one advantageous embodiment, the application is implemented by spraying, nebulisation or atomisation of the said solution or emulsion. It is thus possible to spray, nebulise or atomise the solution or emulsion on the surface of the grains, for example while they are being transported on a conveyer belt, on which the product must pass, whether or not it is treated according to the invention. There is thus a saving in special heavy equipment for executing the invention.
Details and particularities of the method according to the invention are also to be taken from Claims which follow.
The product and the method according to the invention will now be illustrated by means of non-limiting examples.